Physical Sciences Research Highlights

Sugar Hitches a Ride on Organic Sea Spray

Floater Follow-up Scientists are learning more about chemical interaction mechanisms that may be responsible for the high amount of sugar-like material found in sea spray produced from ocean bubbles that burst and launch the tiny particle hitchhikers into the atmosphere. (See sidebar: Fatty acids and saccharides, ahoy!) Ultimately, they will learn how these particles impact the brightness of the cloud layers formed above the ocean, which have an effect on the Earth’s climate. Graphic by Nathan Johnson at PNNL. Enlarge Image.

Exiting
the airport, travelers catch a taxi, Uber, or bus ride to their next stop. Seafaring
sugar molecules floating near the ocean's surface take a similar tack. Instead
of taxis, they hitch a ride on oily molecules floating by.

Results: Researchers at Pacific Northwest
National Laboratory, Montana State University, and Los Alamos National
Laboratory found this "sticky" strategy not only shields these molecules from
their soluble nature, it explains the discrepancies between models that predict
sea spray's organic enrichment and the actual measurements of sea spray aerosol
composition.

The study's findings, published in Geophysical Research Letters, may
explain how so many soluble sugars find their way into sea spray, and provide
clues to how they may affect the amount of sunlight reflected by sea-spray-seeded
clouds.

Why It Matters: Sugar molecules (saccharides) are normally soluble in water. Yet,
somehow, they make their way into sea-spray particles that are tossed into the
atmosphere by breaking waves, eventually helping seed low-lying marine clouds.
These clouds have a large role in the climate because they regulate the amount
of sunlight that hits the ocean surface—the largest heat sink on the planet. By
solving the mechanistic mystery by which sugars and other organic matter in sea
spray aerosol, such as these sugars, is emitted to the atmosphere, scientists
will be better able to simulate its impacts on the climate. This information
allows better estimations of the amount of sunlight that is reflected by
clouds, which has a cooling effect on the Earth.

Methods: Scientists are interested in the composition of particles
tossed into the atmosphere by sea spray, a large source of water vapor helping
form clouds. What are these particles that affect marine clouds? Researchers
who analyze sea spray samples collected onboard ships found that they contain a
large amount of saccharides (sugar-like molecules). However, because
saccharides easily dissolve in water, it was unclear how this material survived
to enter the spray.

A team of researchers investigated
the water surface interactions between saccharides and fatty acids—oily
molecules that are insoluble in water. Montana State University researchers and
EMSL staff performed spectroscopy experiments at the EMSL facility and showed
that saccharides can adsorb (stick) to the bottom of a layer of fatty acids
that coat the water surface. This adsorption
causes an increased amount of saccharide molecules to be present at the
surface. When the layer of fatty acids was not present, the saccharide
molecules dissolved in the water. Because sea spray aerosol forms from the
surface layer of ocean water, mechanisms similar to the one investigated in
this study could increase the amount of organic matter emitted in sea spray.

Using a model developed
at PNNL and Los Alamos, researchers tested the sensitivity of modeled sea spray
composition to this mechanism. They found that if the molecules adsorb strongly
enough, the amount of organic matter emitted in sea spray could be
substantially increased. These organic
matter emissions could potentially impact the amount of sunlight that is
reflected by clouds that are influenced by this spray.

What's Next? Researchers suggest further experiments to test interactions of additional
organic molecules that reflect the range of chemistry occurring in the ocean's
surface waters. They want to identify whether the interactions studied in
surface films will affect the composition of artificially generated sea spray
aerosol.

Additional Information

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Fatty acids and saccharides, ahoy!

In one sentence: Hitching a ride on fatty molecules, a
"sticky" strategy shields sugary molecules from their soluble nature, and may
explain the discrepancies between models and actual measurements of sea spray
aerosol composition.

When tiny sea
critters called phytoplankton die, their remains break down into individual
molecules, including fats and sugars, which float in the water near the ocean's
surface. (See graphic.) Scientists are learning about the interactions between
these kinds of molecules by studying a simpler chemical mixture. These
molecules are negatively charged fatty acids, and positively charged sugars, or
saccharides. Saccharides are the building blocks of more complex
polysaccharides such as starches, the material that makes up the shells of
crabs and shrimp, as well as other similar hard structures in other organisms.
The fats float to the water's surface where they may bind (stick) the sugary
molecules dissolved in the water below.